It is known to detect toxic gases using chemical cassettes having a paper tape impregnated with a specific chemical material; the paper is usually absorbent and is impregnated with the chemical material by dipping it into a trough of a solution of the material. The tape is subsequently dried and installed in a cassette. During use, the paper tape is placed in an analyser that draws gas from the atmosphere being monitored through the tape. The chemical material on the tape is chosen to react with the target gas and to change colour. The degree of colour change is a measure of the concentration of the target gas in the atmosphere being monitored. The analyser detects the change of colour and calculates the gas concentration by comparison to a table of known gas responses that has been pre-programmed into the analyser. The tape is advanced periodically to bring a fresh length of tape to the position within the analyser through which the target gas is drawn to expose a fresh length of tape to make a further measurement. One major advantage of the tape system is that a permanent visual record exists of positive gas responses as seen by the visible colour change of the tape.
The present system gives rise to several problems. Firstly, the material supported on the tape that changes radiation absorption properties on exposure to the target gas (hereinafter referred to as the “colour-change material”) can give off a gas during reaction with the target gas. This evolved gas can lead to inaccuracies in the measurement.
Secondly, the tape and the colour-change material can degrade over time in response to temperature, humidity and/or light. It therefore has a limited shelf life and so only a limited stock can be held by the user.
It is critical that the tape is uniform along its length to ensure a uniform response to a given amount of target gas. Therefore, inconsistencies in the tape must be avoided. Also, if a paper tape includes variations in thickness, e.g. “lumps” within the paper material of the tape, this can cause the analyser to jam. Furthermore “lumps” in the tape can result in variable gas flow through the tape and therefore result in further inaccuracies in the measurement, ultimately possibly causing a false alarm or an alarm indicting that the detection system is not operating properly (fault alarm).
The colour-change material can be affected by gases in the atmosphere being monitored other than the target gases. Often, water vapour can change the response of the colour change materials to a target gas, leading to inaccuracies in the measurement.
It is an object of the present invention to overcome or alleviate some or all of the above problems and/or to provide a system bearing colour-change materials that can be used to detect two or more gases.